1. Field of the invention
The present invention relates to a system for transmitting audio signals and/or video signals as a digital signal in such applications as a digital video cassette recorder (VCR) whereby audio signals and/or video signals are recorded and reproduced as digital signals, and also to a method thereof.
2. Description of the prior art
Devices for transmitting audio signals and/or video signals via a digital signal transmission path are being continually developed. Transmitting audio signals and/or video signals as a digital signal, however, requires sending and receiving to be synchronized to the processing speed of the apparatus, and therefore requires a transmission path capable of isochronous transmission. A bus connection is even more preferable considering the potential need for two-way communications on a single transmission path whereby plural devices can receive a signal transmitted from a single device.
The Institute of Electrical and Electronic Engineers, Inc. (IEEE) is currently studying a next-generation high speed serial bus protocol under the title P1394 (see "High Performance Serial Bus"). Under the proposed IEEE P1394 standard, isochronous transmission data, including audio signals, video signals, and other real-time data, can be transmitted by isochronous transmission using isochronous packets, which are sent and received every 125 .mu.sec (=1 cycle) to achieve isochronous transmission.
The isochronous transmission control method of IEEE P1394 is described next. When the bus is initialized according to IEEE P1394, a node identifier is automatically assigned to each device connected to the bus (each `node`) as a means of identifying each node. A maximum 64 isochronous packets per cycle can also be sent over the bus. As a result, each isochronous packet is also assigned a channel number ranging in value from 0 to 63 to identify each isochronous packet. To achieve isochronous transmission on plural channels, one of the plural nodes connected to the bus is used for isochronous transmission management; this node is called the "bus manager" below.
The bus manager manages the channel numbers used for isochronous transmission, and the time remaining in each cycle usable for isochronous transmission. The time sharing rate, or the time slot width, required for each node to transmit an isochronous packet in one cycle is referred to as a bandwidth below. To achieve isochronous transmission, the bus manager must reserve the channel number and the bandwidth to be used. It should be noted that communications not essential to isochronous transmission and information that does not require isochronous transmission are transmitted by asynchronous transmission using asynchronous packets. Asynchronous communication is accomplished using cycle time not used for isochronous transmission.
The bus is also immediately reinitialized whenever a node is connected or disconnected from the bus, or whenever any node on the bus is turned off, to enable active bus configuration.
The first problem addressed by the present invention is described next.
When the IEEE P1394 high performance serial bus is applied to isochronous transmission between consumer audio-visual (A/V) devices using the conventional isochronous packet described above, it is not possible for the node receiving the isochronous packet to identify the node sending that isochronous packet.
Because of this node identification problem, the node receiving the isochronous packet cannot request the node sending the isochronous packet to continue isochronous packet transmission when it is necessary to prevent interruption of isochronous transmission due to an unexpected user action, and it is therefore not possible to set the transmission node to a protected state.
This is described below referring to a system comprising nodes A, B, and C with node B assumed to be receiving and recording the isochronous packet sent by node A. If the user then performs some action causing node C to transmit an isochronous packet, node C must request node A to stop transmitting the isochronous packet. If node A responds to this request by stopping transmission, the recording operation of node B will be interrupted. It is therefore possible by this conventional data transmission method to interrupt the transmission of isochronous packets between communicating nodes when one node not associated with that isochronous packet transmission is accidentally or improperly operated.
The second problem is described next. As described above, the IEEE P1394 protocol enables plural channels of real-time data to be output during one cycle. It is therefore necessary for the receiver node(s) to determine the channel numbers of the real-time data that should be received by that node. One method of enabling the receiver node to determine the channel numbers to be received is for the user to inform the receiver node of the channel numbers to be received. To do this, however, the user must determine and inform the receiver node of the channel numbers of the real-time data that should be received, and this increases the burden on the user.